Electromagnetic testing emt-rft chapter 8b

Charlie Chong/ Fion Zhang

Electromagnetic TestingMagnetic flux leakage / Eddy current/ MicrowaveChapter 8 – Remote Field Testing 远场涡流Reading 216th Feb 2015My ASNT Level III Pre-Exam Preparatory Self Study Notes





Charlie Chong/ Fion Zhang http://www.themalaysianinsider.com/world/article/sinopec-oil-pipeline-blast-kills-44-in-eastern-china

Charlie Chong/ Fion Zhang http://www.themalaysianinsider.com/world/article/sinopec-oil-pipeline-blast-kills-44-in-eastern-china

Charlie Chong/ Fion Zhang http://www.s1979.com/tupian/china/201311/22107372922_8.html


Fion Zhang at Shanghai2015 February

Charlie Chong/ Fion Zhang Shanghai 上海



Chapter Eight:Remote Field Testing Reading Session 2

https://www.nde-ed.org/EducationResources/CommunityCollege/Other%20Methods/RFT/RFT_Intro.htm


Reading 1: Remote Field Testing RFTRemote Field Testing or "RFT" is one of several electromagnetic testing methods commonly employed in the field of nondestructive testing. Other electromagnetic inspection methods include magnetic flux leakage, conventional eddy current and alternating current field measurement testing. Remote field testing is associated with eddy current testing and the term "Remote Field Eddy Current Testing" is often used when describing remote field testing. However, there are several major differences between eddy current testing and remote field testing which will be noted in this section.

https://www.nde-ed.org/EducationResources/CommunityCollege/Other%20Methods/RFT/RFT_Intro.htm


RFT is primarily used to inspect ferromagnetic tubing since conventional eddy current techniques have difficulty inspecting the full thickness of the tube wall due to the strong skin effect in ferromagnetic materials. For example, using conventional eddy current bobbin probes to inspect a steel pipe 10 mm thick (such as what might be found in heat exchangers) would require frequencies around 30 Hz to achieve the adequate I.D. to O.D. penetration through the tube wall. The use of such a low frequency results in a very low sensitivity of flaw detection.

The degree of penetration can, in principle, be increased by the use of:

■ partial saturation eddy current probes, ■ magnetically biased probes, and ■ pulsed saturation probes.

However, because of the large volume of metal present as well as potential permeability variations within the product, these specialized eddy current probes are still limited in their inspection capabilities.

http://www.shipstructure.org/pdf/91symp22.pdf


The difficulties encountered in the testing of ferromagnetic tubes can be greatly alleviated with the use of the remote field testing method. The RFT method has the advantage of allowing nearly equal sensitivities of detection at both the inner and outer surfaces of a ferromagnetic tube. The method is highly sensitive to variations in wall thickness and tends to be less sensitive to fill-factor changes between the coil and tube. RFT can be used to inspect any conducting tubular product, but it is generally considered to be less sensitive than conventional eddy current techniques when inspecting nonferromagnetic materials.

http://www.shipstructure.org/pdf/91symp22.pdf


RFT Theory of OperationA probe consisting of an exciter coil and one or more detectors is pulled through the tube. The exciter coil and the detector coil(s) are rigidly fixed at an axial distance of two tube diameters or more between them. The exciter coil is driven with a relatively low frequency sinusoidal current to produce a magnetic field.


These eddy currents, in turn, produce their own magnetic field, which opposes the magnetic field from the exciter coil. Due to resistance in the tube wall and imperfect inductive coupling, the magnetic field from the eddy currents does not fully counterbalance the magnetic exciting field. However, since the eddy current field is more spread out than the exciter field, the magnetic field from the eddy currents extends farther along the tube axis. The interaction between the two fields is fairly complex but the simple fact is that the exciter field is dominant near the exciter coil and the eddy current field becomes dominant at some distance away from the exciter coil.


Keywords:The interaction between the two fields is fairly complex but the simple fact is that:

The exciter field is dominant near the exciter coil and The eddy current field becomes dominant at some distance away from the

exciter coil.


The receiving coils are positioned at a distance where the magnetic field from the eddy currents is dominant. In other words, they are placed at a distance where they are unaffected by the magnetic field from the exciter coil but can still adequately measure the field strength from the secondary magnetic field. Electromagnetic induction occurs as the changing magnetic field cuts across the pick-up coil array. By monitoring the consistency of the voltage induced in the pick-up coils one can monitor changes in the test specimen. The strength of the magnetic field at this distance from the excitation coil is fairly weak but it is sensitive to changes in the pipe wall from the I.D. to the O.D.


Electromagnetic Testing

1. Conventional ECT 2. Remote Field ECT

4. Magnetic Flux Leakage ECT3. Near Field ECT

http://www.olympus-ims.com/cs/ms-5800-tube-inspection/

5. ACFM ECT


Electromagnetic Testing – Conventional ECT Eddy current testing is a noncontact method used to inspect non-ferromagnetic tubing. This technique is suitable for detecting and sizing metal discontinuities such as corrosion, erosion, wear, pitting, baffle cuts, wall loss, and cracks in nonferrous materials.


Two coils are excited with an electrical current, producing a magnetic field around them. The magnetic fields penetrate the tube material and generate opposing alternating currents in the material. These currents are called eddy currents.

Any defects that change the eddy current flow also change the impedance of the coils in the probe.

These changes in the impedance of the coils are measured and used to detect defects in the tube.


Electromagnetic Testing – Remote Field ECT

Remote field testing (RFT) is being used to successfully inspect ferromagnetic tubing such as carbon steel or ferritic stainless steel. This technology offers good sensitivity when detecting and measuring volumetric defects resulting from erosion, corrosion, wear, and baffle cuts.



Electromagnetic Testing – Near Field ECT

Near field testing (NFT) technology is a rapid and inexpensive solution intended specifically for fin-fan carbon-steel tubing inspection. This new technology relies on a simple driver-pickup eddy current probe design providing very simple signal analysis.

NFT is specifically suited to the detection of internal corrosion, erosion, or pitting on the inside of carbon steel tubing. The NFT probes measure lift-off or "fill factor," and convert it to amplitude-based signals (no phase analysis). Because the eddy current penetration is limited to the inner surface of the tube, NFT probes are not affected by the fin geometry on the outside of the tubes.



Electromagnetic Testing – Magnetic Flux Leakage ECT

Magnetic flux leakage (MFL) is a fast inspection technique, suitable for measuring wall loss and detecting sharp defects such as pitting, grooving, and circumferential cracks. MFL is effective for aluminum-finned carbon steel tubes, because the magnetic field is almost completely unaffected by the presence of such fins.



Electromagnetic Testing – ACFM ECTIn a basic alternating current field measurement system, a small probe is moved along the toe of a weld. The probe contains an exciter coil, which induces an AC magnetic field in the material surface aligned to the direction of the weld. This, in turn, causes alternating current to flow across the weld. – ASTM E2261-12



IRIS UTThe ultrasonic IRIS option is used to inspect a wide range of materials, including ferrous, nonferrous, and nonmetallic tubing. This technique detects and sizes wall loss resulting from corrosion, erosion, wear, pitting, cracking, and baffle cuts. Olympus digital IRIS inspection technology is used extensively as a prove-up technique for remote field testing, magnetic flux leakage, and eddy current inspections.



Electromagnetic Testing – Conventional ECTEddy current testing is a non-contact method for the inspection of nonferromagnetic tubing. This technique is suitable for the detection and sizing of metal discontinuities such as corrosion, erosion, wear, pitting, baffle cuts, wall losses and cracks in nonferrous materials.

http://absolutende.com/en/solutions/tube


Electromagnetic Testing – Remote Field ECT



IRIS UT



IRIS UT



The Zones

These eddy currents, in turn, produce their own magnetic field, which opposes the magnetic field from the exciter coil. Due to resistance in the tube wall and imperfect inductive coupling, the magnetic field from the eddy currents does not fully counterbalance the magnetic exciting field.

Eddy current field is more spread out than the exciter field, the magnetic field from the eddy currents extends farther along the tube axis.


Direct Couple ZoneThe region where the magnetic field from the exciter coil is interacting with the tube wall to produce a concentrated field of eddy currents is called the direct field or direct coupled zone. This zone does not contribute a great deal of useful data to the RFT inspection due to problems with rather high noise levels due to the intense varying magnetic field from the excitation coil.

Keywords:High Noise Level.


Transition ZoneThe region just outside the direct couple zone is known as the transition zone. In this zone there is a great deal of interaction between the magnet flux from the exciter coil and the flux induced by the eddy currents. As can be seen in the graph, the interaction of the two opposing fields is strongest near the ID of the tube and fairly subtle at the OD of the tube. The "resultant" field strength (the magnetic field at the sum of the two fields) in this region tends to change abruptly on the ID due to the interaction of the fields with differing directional characteristics of the two fields.


The receiver coil's signal phase, with respect to the exciter coil, as a function of distance between the two coils is also shown in the graph. When the two coils are directly coupled and there is no interference from a secondary field, their currents are in phase as seen at location zero. In the transition zone, it can be seen that the phase swiftly shifts, indicating the location where the magnetic field from the eddy currents becomes dominate and the start of the remote field.


DiscussionSubject: Study on the phase lag of ID/OD amplitudes.


Remote Field ZoneThe remote field zone is the region in which direct coupling between the exciter coil and the receiver coil(s) is negligible. Coupling takes place indirectly through the generation of eddy currents and their resulting magnetic field. The remote field zone starts to occur at approximately two tube diameters away from the exciter coil. The amplitude of the field strength on the OD actually exceeds that of the ID after an axial distance of approximately 1.65 tube diameters. Therefore, RFT is sensitive to changes in material that occur at the outside diameter as well as the inside diameter of the tube.


The amplitude of the field strength on the OD actually exceeds that of the ID after an axial distance of approximately 1.65 tube diameters.

The amplitude of the field strength on the OD actually exceeds that of the ID after an axial distance of approximately 1.65 tube diameters.


RFT ProbesProbes for inspection of pipe and tubing are typically of the bobbin (ID) variety. These probes use either a single or dual excitation coil to develop an electromagnetic field through the pipe or tube. The excitation coils are driven by alternating current. The sensing coil or coils are located a few tube diameters away in the remote field zone. Probes can be used in differential or absolute modes for detection of general discontinuities, pitting, and variations from the I.D. in ferromagnetic tubing. To insure maximum sensitivity, each probe is specifically designed for the inside diameter, composition, and the wall thickness of a particular tube.


RFT Probes


RFT InstrumentationInstruments used for RFT inspection are often dual use eddy current / RFT instruments employing multi-frequency technology. The excitation current from these instruments is passed on to the probe that contains an exciter coil, sometimes referred to as the driver coil. The receiving coil voltage is typically in the microvolt range, so an amplifier is required to boost the signal strength.

Certain systems will incorporate a probe excitation method known as multiplexing. This utilizes an extreme high speed switching method that excites the probe at more than one frequency in sequence. Another method of coil excitation that may be used is simultaneous injection. In this coil stimulation technique, the exciter coil is excited with multiple frequencies at the same time while incorporating filter schemes that subtract aspects of the acquired data. The instrument monitors the pickup coils and passes the data to the display section of the instrument. Some systems are capable of recording the data to some type of storage device for later review.


RFT Instrumentation


RFT Instrumentation


RFT Instrumentation


RFT Instrumentation


RFT Signal InterpretationThe signals obtained with RFT are very similar to those obtained with conventional eddy current testing. When all the proper conditions are met, changes in the phase of the receiver signal with respect to the phase of the exciter voltage are directly proportional to the sum of the wall thickness within the inspection area. Localized changes in wall thickness result in phase and amplitude changes. These changes can be indicative of defects such as cracks, corrosion pitting or corrosion/erosion thinning.


RFT Signal Interpretation


The ID Phase Lag


The Phase LagThe skin depth equation is strictly true only for infinitely thick material and planar magnetic fields. Using the standard depth δ , calculated from the above equation makes it a material/test parameter rather than a true measure of penetration.

(1/e)

(1/e)2

(1/e)3

FIG. 4.1. Eddy current distribution with depth in a thick plate and resultant phase lag.


RFT Reference StandardsReference standards for the RFT inspection of tubular products come in many variations. In order to produce reliable and consistent test results, the material used for manufacturing calibration standards must closely match the physical and chemical properties of the inspection specimen. Some of the important properties that must be considered include conductivity, permeability and alloy content. In addition, tube dimensions including I.D., O.D. and wall thickness must also be controlled.

The type of damage mechanisms that are expected to be encountered must also be carefully considered when developing or selecting a reference standard. In order to get accurate quantitative data, artificial discontinuity conditions are typically machined into the standards that will closely match those conditions that may be found in the tubing bundle.


Reading 2: Remote Field Testing of Ferromagnetic TubesRemote Field Testing (RFT) is the purely magnetic technique useful to detect flaws in materials with permeability sufficient to prevent significant penetration of eddy currents. RFT is primarily used to inspect ferromagnetic tubing since conventional eddy current techniques have difficulty inspecting the full thickness of the tube wall due to a strong skin effect in ferromagnetic materials. Eddy currents are also generated in tested material but particularly in the region near the excitation coils. However, the position of the receiver coils is far enough from the exciters that the influence of eddy currents is negligible. "Remote" magnetic fields are capable of passing through tested material. In penetrating the material, the magnetic field travels along the outside surface and the coils detect disturbances in the flux field in comparison to the primary magnetic field.

http://www.instytutgamma.com.pl/rft.html


These can be very quickly scanned for both internal and external wall loss defects such as a corrosion, erosion, pitting, cracks, and wear scar.

RFT can be used to inspect any conducting tubular product. Evaluating the quality of products by Remote Field Testing method is the most effective for ferromagnetic materials like low carbon steels, chrome molly, duplex in units like:

heat exchangers feedwater heater furnace tubes in vessels Texas Towers Went coolers with fin fan tubes



RFT Expert at Works



The 3D Push Pull High Speed Data Acquisition System consisting of professional Zetec's software allows for the checking of about 400 tubes per shift. Institute Gamma utilizes the highest technology of multi-frequency testing equipment including the Zetec's MIZ-27SI and RFT Amplifier.

Institute Gamma's qualified engineers and technicians perform maintenance and inspection services.

Engineers with competence in maintenance analysis and planning, inspection planning, damage evaluation, material technology and risk based inspection.

NDT senior inspectors on level 2 and 3 in accordance with EN-473/PCN-BIND certification system.

NDT technician on level I and II with ZETEC certificates for RFT method Data analysts for eddy current on level IIA/QDA in accordance with EPRI

S/G certification. Welding inspectors in accordance with European Welding Federation

requirements and EN-719 standard.



Benefits of RFT tube testing are:

Prevention of dangerous leaks Prevention of break down outages Accurate assessment of wall thickness loss up to 0,1mm Recorded measurement data , forecasting of wall loses, predictive

maintenance Quantitative method Detecting abnormalities of used materials RFT doesn't need water and clean surface like the IRIS method RFT is more sensitive and sizes defects more accurately than other

magnetic methods



Remote Field Testing - Data Display



Reading 3: Steam Boiler Inspections Using Remote Field TestingForced outages of steam boilers due to tube leakages remain the leading cause of lost production in plants. Due to hundreds, if not thousands of linear feet of pipe, there is a high potential for failure without notice. One of the biggest challenges for maintenance and operations personnel is the prevention of tube failures in boilers and heat exchangers without causing significant loss to the company. When excursions from normal operating conditions occur, the question must be raised, “if our boiler tubes were damaged by the excursion (for example, overheating, or a condenser tube leak), how do we find out if we have a problem that could lead to failure?”

Boiler operation always involves harsh working conditions. On the fuel side of tubes, high operating temperatures and corrosive by-products from burning fossil fuels or solid waste can seriously degrade the life-expectancy of the boiler tubes. On the water/steam side, there is a high potential for oxidization of boiler tubes due to high temperature steam and the corrosive action of chemicals in the water supply. These conditions may cause metal to overheat, corrosive wall thinning, and localized pitting, any or all of which can lead to premature failure of the tubes, possible injuries to personnel, damage to adjacent tubes and a forced outage.

http://www.power-eng.com/articles/print/volume-115/issue-3/features/steam-boiler-inspections-using-remote-field-testing.html


Fire Boiler



In spite of these adverse operating conditions, boilers have a life expectancy upwards of 30 years and most premature failures are due to conditions that make operational variables deviate from expected parameters.Industrial best practice is to inspect all tubes periodically, checking to ensure that mechanical properties of the materials are intact and that material thickness is within normal expectations. A proper and rigorous inspection regimen will go a long way to reduce the probability of premature boiler failures.

Thomas R. Schmidt of Shell Oil headed the initial development of the remote field testing (RFT) technique for measuring oil well casings. After that, several tools have entered the market using RFT for multiple specialized applications.

The primary benefit of this technique is that it does not require contact with the object under test to measure material thickness and condition. Additionally, a high-quality inspection can be assured without requiring couplant and with minimal surface preparation. RFT also shows high sensitivity to detection of defects on the ID or OD of the tube in question and can measure through non-ferromagnetic coatings, linings and scale.



There are two approaches to doing maintenance of any kind: a preventative approach or a corrective approach (pro-active or reactive). This holds true with boiler inspections. A preventative approach seeks to look at the long-term wear-and-tear tendencies on the equipment, with an eye towards improving operations through improvements to the fuel-air mixture, flow balancing and the creation of a maintenance specification for tube repair or replacement. Undertaking a corrective approach looks to inspect the boiler after a failure to look for collateral damage and to ensure that the failure mechanism has not affected other areas in the boiler.

Of the two, the former is more advantageous from the perspective of being performed before failure, within the context of the firm’s long-range operational plan, resulting in a reduced effect to the operations budget and significantly reduced time lost due to unexpected outages.



Long-term overheating (creep): This type of failure occurs when the operating temperature of the boiler tubes exceeds the operational limits for an extended period of time. These limits are variable based on the tube size and thickness, operating pressures, as well as the tube locations in the boiler and construction materials. Overheating leading to creep damage can be caused by internal deposits, which reduce flow through the tubes or, more commonly, sudden spikes in operating temperature due to increasing load or issues with the temperature control. These reduce the resistance of the tubes.

Figure 1 Long-term overheating (creep)



How to detect creep damage: During the period of long-term overheating, the surface of the tube will develop blisters at the locations subject to the highest temperature and will develop elongated axial cracks. Both of these failures will reduce overall tube thickness and material properties of the tube. Additionally, thick, dark, brittle oxides will appear on the internal and external surfaces of the tube. All of these conditions can be detected through RFT as the changes in the tubes electrical permeability can be easily measured.

How to prevent creep damage: Often, when boilers are operating outside of standard operational parameters, it goes unrecorded or unnoticed by the operator. Either of these issues can lead to premature failure, as the degradation of the tube’s material properties is not being accurately recorded. As such, it is necessary to run periodic performance evaluations on the boiler. Ensuring frequent calibration of the thermostats is the best way to prevent unexpected temperature spikes and to help ensure that the unit is warmed up in accordance with the manufacturer’s recommended specifications. Frequent sampling of the unit’s feed water supply ideally once per shift—will help to ensure that the feed water quality is within the manufacturer’s recommended parameters.



Regular internal flushing of the boiler tubes will ensure removal of any material deposits clogging the tubes. Any deposits must be measured frequently, and chemical cleaning is recommended when the deposit density exceeds 15 g/sq.ft. Cleaning becomes mandatory as the density reaches 30 g/sq/ft. Frequent drum inspections are mandatory according to the maintenance manual of the manufacturer. Depending on the hours of service, it is necessary to determine how well the equipment is working, the effectiveness of the water treatment and that there are no failure mechanisms affecting the internal surface, mainly in the area of water-steam interface line.

Lastly, it is necessary that all spare parts used in maintenance or repairs are correct to the manufacturer’s specifications. It should be noted that when any tube fails as a result of creep damage, there will be a rupture with slightly rounded edges and jagged edges with cracks or breaks in the vicinity of the rupture. A thick, fragile layer of magnetite will appear near the failure, indicative of long-term overheating.



Short Term Overheating: Most often, these failures occur when the tubes are left without sufficient cooling and occur in short order. Failures caused by short-term overheating frequently occur at the top of the tubes or close to the steam collector. If the failure occurs in a single tube and if surrounding tubes have no appearance of alteration, it suggests that the failed tube was at least partially obstructed, causing the temperature to rapidly exceed material limits, causing an explosion or leak in the tube.

Figure 2 Short-term overheating



How to detect short-term overheating: As these failures occur rapidly, it is recommended that the tubes be inspected visually through the inspection ports during start-up. If red spots suddenly appear on a tube, it is a signal that the tube may be plugged. This type of inspection is necessary after chemical cleaning, tube replacement or re-commissioning after a long period of dormancy.

How to prevent short-term overheating: As a result of the rapid occurrence of this type of failure, it is not readily detectable through non-destructive testing methods. The best way to prevent it is to flush the tubes with water to ensure all obstructions are removed prior to startup and by ensuring that the purge and bottom headers are open as the pressure is increased. This type of failure can be recognized by the longitudinal rupture, smooth edges and no loss of wall thickness as the rupture.



Oxygen Corrosion: Oxygen corrosion occurs in a boiler due to small corroded regions which acts as an anode to the rest of the boiler, causing further corrosion. This process is exacerbated by the presence of dissolved oxygen in the boiler system. Ideally, the boiler surface would be covered with a protective layer of iron oxide, preventing the attack of free oxygen in the water supply. The small pits that result from oxygen corrosion can cause significant stress, and will result in the formation of cracks in the region

Figure 3 Oxygen corrosion



How to detect oxygen corrosion: RFT is one of the most effective methods used to detect oxygen corrosion if it is located at the fire side. High tool sensitivity and accuracy (>1/8” diam.) allows for the early detection of initial defects, providing data necessary to determine a repair or replacement protocol before the tube fails. Ultrasonic testing (UT) can also be used, but is limited by significantly slower inspection time and the fact that 100 percent coverage is impossible. The latter increases the risk that serious damage to tubes could go unnoticed.

How to prevent oxygen corrosion: The most effective way to prevent oxygen corrosion in boiler tubes is to prevent oxygen from entering the system in the first place.Oxygen enters a boiler system primarily through three means: air can be trapped during normal operation when internal pressure is less than the ambient atmospheric pressure; when the system is left open for maintenance; and as a result of molecular dissociation of water in the system. Other critical factors are the presence of ambient moisture in the system and, the loss of a passivation layer after chemical cleaning. Eliminating these factors can successfully prevent oxygen corrosion. It is recommended that all metal surfaces be kept dry using positive-pressure inert gas, moisture-absorbing materials, or a continuous flow of dry, dehumidified air (<30 percent).



Caustic Corrosion: Caustic corrosion refers to the corrosive action of sodium hydroxide with a metal and is restricted to: water-cooled tubes in regions of high heat fluctuation; regions with heat transfer in welding rings or other devices that disrupt flow; horizontal or inclined tubes; places with thick internal deposits reducing flow rates. This penetration may be filled with dense corrosion products which sometimes contain magnetite crystals. Most often, the metal surface has a smooth contour and laminations. Sodium hydroxide is added to boiler water in non-corrosive concentrations; however other physical factors tend to concentrate it further, leading to the production of corrosive alkaline in the boiler.


Figure 4 Caustic corrosion


How to detect caustic corrosion: Caustic corrosion is easily detected using non-destructive testing methods, because the affected area is found with reduced wall thickness. If this is in its early stage there may not be any blister, but if the thickness is reduced there is likely to be a blister or deformation in the tube. Remote field testing is the most suitable method. These tools have high sensitivity and inspection speed, allowing for rapid detection of corrosive damage. UT can also be used, once the affected region has been located.

How to prevent caustic corrosion: When sodium hydroxide is present—either by itself, or as a salt-producing alkaline—with a concentration mechanism, there exists the possibility of caustic corrosion. To reduce the likelihood of caustic corrosion damage in a boiler, the amount of free sodium hydroxide available to produce alkaline salts in the condenser water must be controlled at the purification stage. This will prevent nucleate boiling, and the formations of water-level lines. Proper purging will prevent the formation of sludge deposits.



Stress Corrosion: Stress corrosion is caused by a combination of two separate factors: tensile stresses on the pipe caused by internal pressure, or residual stresses induced by improperly-applied heat treatment, or tube bending; and a corrosive material such as sodium hydroxide or chlorine. This combination results in cracking near the stressed region. Stress corrosion usually occurs near welds, or tube bends.


Figure 6 Stress corrosion


How to detect stress corrosion: Stress corrosion displays as cracking near welds subject to tensile stresses. While stress corrosion cracking can be difficult to see, it can be detected visually. Liquid penetrant inspection provides a surer means of detection. Additionally, ultrasonic testing or radiographic testing can detect stress corrosion cracking.

How to prevent stress corrosion: Annealing will relieve residual stresses from welding or bending. Adding phosphates to the operating environment will help prevent the formation of free sodium, reducing the concentration of corrosion products.



Hydrogen Damage: When chemicals are added to boiler water to balance pH, an electrochemical reaction can occur, releasing free hydrogen atoms into the environment. This can cause decarburization, embrittlement and the formation of molecular hydrogen and methane in the steel. Hydrogen damage is restricted to evaporator tubes with pre-existing corrosion problems. Hydrogen atoms diffuse into the steel of the boiler tubes. Some of these atoms bond with either each other, or the carbon in the steel, forming molecular hydrogen or methane. These gasses accumulate until the pressure causes the separation of the metal along the granular borders, producing inter-granular micro-cracks. This in turn reduces the mechanical strength of the tube, which causes it to burst. Any tubes suspected of failing due to hydrogen damage should have samples taken and sent for metallographic analysis in a lab.


Figure 7 Hydrogen damage


How to detect hydrogen damage: Hydrogen damage is hard to detect visually, except in the advanced stages when the pipe has visible cracks. Remote field testing is highly effective in detecting hydrogen damage because the changes in the electrical properties of the material due to hydrogen damage are readily detected.

How to prevent hydrogen damage: The two critical factors in reducing a boiler’s susceptibility to hydrogen damage are the amount of hydrogen available, and the means to increase its concentration. Proper chemical treatment of feed water, combined with a stringent pH control system is the best way to prevent hydrogen damage.



Graphitization: Graphitization is caused by small structural changes of low-carbon steels at moderate temperatures over extended periods of time. Graphitization causes the decomposition of pearlite in ferrite, weakening the steel. The extent of the decomposition is dependent on the temperature. This phenomenon generally occurs due to long-term overheating, during which, the graphite nodules are linked to each other, reducing the resistance to internal pressure, causing the metal to tear.


Figure 8 Graphitization


How to detect graphitization: Graphitization occurs internally, with the graphite detaching from the steel reducing the total wall thickness. As with hydrogen damage, the electrical properties of the material changes when graphitization is present. For this reason, remote field testing is the most effective means of detection.

How to prevent graphitization: The primary concern with respect to the susceptibility to embrittlement due to graphitization is tube quality. Low carbon content steel is more susceptible to graphitization and long-term overheating is liable to exacerbate the problem. A good metal passivation program and treating the boiler feed water with phosphate will reduce the probability of graphitization.



Fire Side Corrosion: Most fuel components can cause corrosion on boiler tubes. Due to incomplete combustion, deposits of combustion residue can change the heat transfer characteristics with potentially severe effects on system efficiency. Most solid fuels contain 10 to 20 percent ash that remains in the boiler after combustion, leading to lost heat transfer and corrosion. While liquid fuels do not exceed 2 percent ash, they do contain elements such as vanadium and sodium.


Figure 9 Fire-side corrosion


How to detect fire side corrosion: When the surface of the boiler tubes are exposed to combustion gases, the damaged area of the surface will change color. Long-term corrosion of this type affects the permeability and conductivity of boiler tubes, as well as causing pitting.

How to prevent fire side corrosion: Fuel selection is of primary concern in addressing the issue of fire-side corrosion. Fuels should be selected containing minimal corrosive agents such as sulfur, sodium and calcium. Second, optimizing the combustion quality through control of temperature, fuel-air mixture, and air balancing will reduce the ash deposits in tubes. Thermocouples should be installed throughout the boiler to indicate when heat transfer is outside optimal operating range, which could be indicative of ash deposits on the tubes. Third, continuous cleaning programs should be implemented in the boiler.



The probability of steam boiler failure is dependent on numerous operational and maintenance factors. The implementation of preventative inspection and tube profile measurements will help to ensure that boilers are active for their expected 30-year average operational lives. An active regime of preventative maintenance will be effective at reducing the financial impact of unexpected shut-downs due to boiler tube failures.

Operations staff need to be trained in preventative maintenance procedures and processes. Often, when boiler tubes undergo catastrophic failure, adjacent tubes are often damaged as well, increasing the outage time. Periodic inspections and follow-up preventative maintenance is necessary to ensure the boiler system remains in good repair, and potentially extend its life-expectancy. With this in mind, we can conclude that periodic boiler inspection is a vital part of any maintenance regime, and that remote field testing is the optimal means for early detection of most causes of boiler tube failure.

Author: Mynor Celis, currently is Marketing Manager for Latin America for Russell NDE Systems, Edmonton, AB. Canada, He has previous experience as operation manager in a coal-fired power plant, having responsibility for the operation of the boiler, water andwastewater systems, high and low boiler steam pressure. He is a mechanical and electrical engineer with MBA specialization.


More Reading


RFT The Principle: Eddy current testing is one of the most extensively used non-destructive testing (NDT) methods for conductive materials. Remote field eddy current (RFEC) is a type of eddy current NDT, and has drawn more and more attention in the nondestructive testing of ferromagnetic tubular structures. RFEC has remarkable advantages such as almost equal sensitivity to inner and outer defects, easy defect characterization and insensitivity to lift-off or wobble [2]. Remote field eddy current testing mainly depends on indirect-coupled electromagnetic energy, which passes through a pipe wall twice, as shown in Figure 1.

http://www.mdpi.com/1424-8220/14/12/24098/htm#sthash.RTUiWO6G.dpuf


RFT Sensors

http://www.mdpi.com/1424-8220/14/12/24098/htm#sthash.RTUiWO6G.dpuf


Remote Field Through Wall Inspection Technique

The Remote Field Eddy Current (RFEC) inspection technique is a nondestructive method which uses low frequency AC and through wall transmission to inspect pipes and tubes from the inside. The through wall nature of the technique allows external and internal defects to be detected with approximately equal sensitivity.

http://www.physics.queensu.ca/~amg/remote_field.html


The RFEC tool uses a relatively large internal solenoidal exciter coil which is driven with low frequency A.C. A detector, or circumferential array of detector coils, is placed near the inside of the pipe wall, but axially displaced from the exciter by about two pipe diameters.

Two distinct coupling paths exist between the exciter and the detector coils.

The direct path, inside the tube, is attenuated rapidly by circumferential eddy currents induced in the tube's wall.

The indirect coupling path originates in the exciter fields which diffuse radially outward through the wall. At the outer wall, the field spreads rapidly along the tube with little further attenuation. These fields re-diffuse back through the pipe wall and are the dominant field inside the tube at remote field spacing.

Anomalies anywhere in the indirect path cause changes in the magnitude and phase of the received signal, and can therefore be used to detect defects.



Direct / Indirect Paths



Skin effect consideration limits conventional eddy current inspection techniques to inspection of only the surface nearest to the probe. The remote field technique is capable of inspecting the entire wall thickness without the need to use ultra low frequency. Like conventional eddy current techniques, RFEC probes respond well to slits because these interact strongly with eddy currents and produce little perturbation of magnetic flux. Although RFEC probes have been used for well casing inspection for many years, it is a rather complex phenomenon. The interaction with defects is now well understood, thanks to the anomalous source defect models and computer animations that we have developed.

Keywords:The remote field technique is capable of inspecting the entire wall thickness without the need to use ultra low frequency.



There is great interest in the detection of stress corrosion cracking (SCC) in pipelines, but tests of ultrasonic tools in gas pipelines have not been entirely satisfactory. There is therefore continued interest in the potential application of RFEC techniques. RFEC probes are already used for commercial inspection of heat exchanger and pressure tubes. There are several other important potential applications for RFEC techniques including the inspection of water and gas distribution lines. These have elbows and tees which are difficult for any other tool to negotiate. We are collaborating on this application as also on the potential use of RFEC for inspection of water supply pipes and lined gas lines.



RFT

http://www.techcorr.com/services/Inspection-and-Testing/Remote-Field-Testing.cfm

Charlie Chong/ Fion Zhang http://www.techcorr.com/services/Inspection-and-Testing/Remote-Field-Testing.cfm

Charlie Chong/ Fion Zhang http://www.techcorr.com/services/Inspection-and-Testing/Remote-Field-Testing.cfm


Tube Cleanliness is as important for the process reasons (i.e. heat transfer) asit is for the Remote Field inspection. Inspections that go the smoothest are ones where the tubes are adequately cleaned prior to the inspection. Not only does this save inspection time and money, but the data acquired from clean tubes VS dirty tubes make the inspection much more accurate. Non-relevant indications can occur from Iron deposits, calcium deposits, etc. These non-relevant indications can mask real defects located underneath.

So how can you tell when the tubes are cleaned enough for a Remote Field inspection? We have developed a “Dummy” probe chart that customers can use to build probe heads to check for tube cleanliness. These probes can be made to screw on to hydro-blasters lance’s and used after the cleaning process is complete to make sure there is proper clearance for the Eddy Current probe.

http://www.techcorr.com/services/Inspection-and-Testing/Remote-Field-Testing.cfm


More Reading:

1. http://www.igcar.ernet.in/benchmark/Tech/20-tech.pdf2. http://www.imtt-usa.com/Publications/2006/Final%20JCAA_2006%20Paper%20-

%20RFEC%20military%20and%20commercial%20platform%20applications.pdf3. http://www.imeko.org/publications/tc10-2014/IMEKO-TC10-2014-010.pdf


ass

Engineering

Electromagnetic testing emt-rft chapter 8b